An important aspect of most methods for compressing sequences of digital video images is motion analysis. In typical motion analysis, a target image in a sequence of images is divided into a plurality of blocks of pixels. A target region comprising at least one pixel block is selected from the target image. The image preceding the target image is analyzed in order to locate a region of the preceding image which closely matches the target region of the target image. Block matching techniques based on mean-square-error or mean-absolute-error are typically used to locate the region of the preceding image which closely matches the target region of the target image. If no motion has occurred, the preceding and target regions will have the same coordinates within the image frame. If motion has occurred, the preceding region will be offset or translated relative to the target region by an amount which is represented by a displacement vector. This process is typically repeated for each target region in the target image, thereby yielding a displacement vector for each target region. These displacement vectors are thereafter applied to a previous reconstructed image to form a predicted image. An error image is formed from the difference (residual errors) between the target image and the predicted image. The displacement vectors and the error image are then encoded for subsequent use by a decoder in decoding the compressed digital video signal. A known motion compensation system is described in U.S. Pat. No. 5,134,478 to Golin, entitled "Method And Apparatus For Compressing And Decompressing A Digital Video Signal Using Predicted And Error Images", the contents of which is hereby incorporated herein by reference.
Pyramidal encoding may be employed to encode error images that result from motion analysis. A pyramidal encoding system that would be suitable for encoding such error images is described in the background portion of parent application Ser. No. 07/813,884, filed Dec. 23, 1991, the contents of which are incorporated herein in their entirety by reference. As discussed in the parent application, pyramidal encoding generally provides an effective means for encoding images. The low frequency components of an image are encoded at low resolution and higher frequency components are encoded at higher levels of resolution or at the full resolution level. Since most of the energy of the image is usually at low frequency, most of the image is encoded with relatively few bits. In such cases, full resolution encoding is needed only to sharpen up the image.
It is an object of the present invention to provide an improved system for encoding images, such as error images that result from motion analysis, using pyramidal encoding.